The invention relates generally to fiber optic sensing systems and sensing methods for harsh environments.
Steady and transient temperature measurements are required in various industrial applications, including extremely harsh environments such as turbine engines, combustion cells, and power plants. Non-limiting examples of harsh environments include coal gasifiers and radiant syngas cooler vessels where the transient temperature typically ranges from 1000° F. to 3000° F. (˜537° C. to 1648° C.), with a pressure greater than 500 psi (˜3.45 MPa). Conventional sensors are often difficult to use in such harsh environments due to the high temperatures, the presence of highly corrosive agents (H2S, SO2, H2O), the electromagnetic interference that may be present in these environments, or combinations thereof.
HTFBG (high temperature fiber Bragg gratings) are highly desirable for multi-point temperature profile measurements due to their advantages in low mass, low specific heat, multiplexing, multi-point distribution, and electromagnetic interference immunity. However, the operation within a gasifier environment, characterized by high temperature, pressure, turbulence, and corrosion, shortens the lifetime of the fiber sensors. In harsh environments, the fiber HTFBG sensors have to be protected not only for providing a desirable surviving rate but also for providing reliability in these environments.
For high-temperature, high-pressure, high-radiation, high-electromagnetic interference, and high-corrosive industrial environments, such as gas/steam turbines, engines, gasifiers, and nuclear reactor vessels, there is currently no practical solution for using the fiber sensors without the sensors experiencing detrimental effects. There are no commercially available fiber temperature sensing cables and systems that can be installed in a harsh environment such as a gasifier or radiant syngas cooler vessel where the steady and transient temperatures could be higher than 2000° F. (˜1093° C.) and pressures greater than 500 psi (˜3.45 MPa).
A bare fiber sensor cannot be installed in a harsh environment. This is not only due to its fragility but also due to the detrimental effect of corrosive gases, moisture, and acidic and alkaline chemicals attacking the fiber sensor. Polymeric coatings such as acrylate, polyimide, silicone, and carbon, are commonly used fiber cable packaging materials that could allow the fiber and fiber sensors to be deployed in a mild environment where the temperature, pressure, corrosion, and moisture are not of significant concern. Metalized fibers and metal-coated fiber sensors are advanced solutions used for harsh environment applications due to high mechanical strength being provided by the metal coating material. The metal coating materials, such as Al, Ni, TiNi, and Au, allow the fiber or fiber sensor to tolerate temperatures up to 1000° F. (˜537° C.). Above 1000° F., however, the mismatched thermal expansion properties that induce interfacial stress and strain between the fiber material and metal coating material quickly degrade the fiber sensor performance and cause poor sensor reliability.
HTFBG sensors have to be packaged before they are installed in or embedded into a structure. Potentially, the packaging of the sensor could protect the sensor from damage due to the hazardous environment and raise the surviving rate of the sensor during the installation and service life. For packaged fiber HTFBG sensors, durability and life span of the fiber HTFBG sensors are not only dependent on the fiber HTFBG sensor itself but also on the packaging materials and package methods.
The most commonly used packaged fiber temperature sensors, such as internal and external Fabry-Perot interferometer-based devices, are metal/ceramic packaged point sensors. It is difficult to cascade and distribute these fiber temperature sensors in a large-scale industrial environment where the temperature is normally higher than 1000° F. (˜537° C.) and the pressure greater than 500 psi (˜3.45 MPa). The limitations are due use of polymer-based package materials, use of adhesive materials, or mismatched thermal and mechanical properties with metal and ceramic material-based packages.
Most commonly used package materials include either stainless steel or glass/ceramic capillaries or tubes. The fiber and fiber sensors are sealed inside the capillary or tube for protection. For sensing applications such as downhole, borehole, and wellbore, oil, gas and geothermal wells, the fiber sensor sealed capillary or tube is embedded into a structure. And the capillary or tube either needs to either be filled with low-compressibility liquid or solid for reducing vibration attenuation for external pressure detection or be filled with high-thermal conductive liquid for external temperature detection. The temperature limits for packaged sensors to be operated in such harsh environments are typically between 400° F. (˜204° C.) and 600° F. (˜315° C.).